In a studless tire which is used for traveling on a snowy road, there is generally employed a block pattern in which a lot of blocks are arranged on a tread surface. This is because traction (a tractional force) is enhanced by a snow column shear stress in a lateral groove which sectionalizes the blocks, thereby achieving improvement of the on-snow traveling performance. Further, during travel on a snowy road, a so-called wandering phenomenon tends to be generated, the wandering phenomenon being caused by vehicle meandering which is generated by the tire getting into a rut. Therefore, a method which can improve the wandering resistance is desired in this kind of tire.
In Patent Documents 1 to 4, there is described a pneumatic tire in which a shoulder land portion positioned in an outermost side in a tire width direction of a tread surface is constructed by block rows, and a plurality of projections are provided in a groove bottom of a shoulder lateral groove which sectionalizes the block rows. However, in order to improve the wandering resistance, it is very important to promote escape from the rut. On the contrary, it is thought that the tires described in the above documents do not generate an operation which is enough to make the tire escape from the rut due to a position where the projection is provided and a shape of the projection.